1,2-Bis(acyloxyates) are useful intermediates for organic synthesis. For example, 3,4-diacetoxy-1-butene is used in the production of vitamin A acetate, Paust, J., Pure and Appl. Chem., 63, 45 (1991). The 1,2-bis(acyloxyates) are the bis-esters of 1,2-dihydroxy compounds, or, alternatively, 1,2-diol dicarboxylates. Of particular interest is the 1,2-diacetoxy esters having the general formula as shown in structure (I). ##STR1##
In the preparation of 1,2-diacetoxy esters by acetylation of epoxides with acetic anhydride various forms or combinations of amine catalysts have been used. For example, a study by Shvets and Al-Wahib discloses that 1,2-diacetoxyethane can be prepared from ethylene oxide with acetic anhydride in the presence of pyridine and that the reaction proceeds by the intermediacy of N-(.beta.-acetoxyethyl)pyridinium acetate. The reaction produces yields ranging from 45 to 93.5 percent. The yields decrease with increasing pyridine concentration at 0.05 to 1.00M. Shvets, V. F. and Al-Wahib, I., Kinet. Katal., 16(3), 785-8 (1975).
In another study by Shvets and Al-Wahib the nucleophilic catalysis of ethylene oxide is disclosed. The reaction of ethylene oxide with acetic anhydride is catalyzed by (CH.sub.3 CH.sub.2).sub.4 N.sup.+ X.sup.- (X=Cl, Br, I) and proceeds by attack of the halide anion on ethylene oxide to form the 2-haloethyl acetate and (CH.sub.3 CH.sub.2).sub.4 N.sup.+ OAc.sup.-. The latter product continues to catalyze the reaction of ethylene oxide with acetic anhydride and also reacts with the 2-haloethyl acetate so that both processes lead to 1,2-diacetoxyethane. Product yields are typically low when using tetraalkylammonium halides. Shvets, V. F. and Al-Wahib, I., Kinet. Katal., 16(2), 425-30 (1975).
As taught by Swindell, the nucleophilic catalysis of an epoxycyclooctane derivative with acetic anhydride catalyzed by 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) and LiCl in tetrahydrofuran produces the corresponding 1,2-diacetoxy ester at a 70 percent yield, Swindell, C. S. and Patel, B. P., J. Org. Chem., 55, 3 (1990). Fraser-Reid discloses that the reaction of epoxy-pyranosides with acetic anhydride catalyzed by (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2).sub.4 N.sup.+ OAc.sup.- opens the epoxy ring to prepare the corresponding 1,2-diacetoxy ester, Fraser-Reid, B. and Rahman, Md. A., J. Amer. Chem. Soc., 107, 5576 (1985).
Tetraalkylammonium acetates are produced by alkylation of a tertiary amine to form a quaternary ammonium salt in which the counterion is exchanged for acetate. Generally, this process requires isolation or purification of the intermediates. Further, the tetraalkylammonium acetates are not thermally stable. For example, tetrabutylammonium acetate rapidly decomposes upon heating over a range of 25.degree.-400.degree. C. At 165.degree. C., tetrabutylammonium acetate retains 90% of its mass, at 190.degree. C. 50% of its mass and at 230.degree. C. 8% of its mass. The thermal instability of tetraalkylammonium acetates decreases catalyst lifetime and hinders the ability to easily recycle the catalyst.
The acid catalyzed ring opening of epoxides in the presence of acetic anhydride is also well known. For example, the acetylation of 3,4-epoxy-1-butene in the presence of acetic anhydride produces 3,4-diacetoxy-1-butene as disclosed by Evans, R. M., Fraser, J. B. and Owen, L. N., J. Chem Soc., 248 (1949). A 70 percent yield is obtained when using hydrochloric acid as the catalyst, while only a 39 percent yield is obtained using anhydrous zinc chloride as the catalyst. Another example of a Lewis acid catalyzed ring opening of an epoxide is disclosed by Ali, S. and Bittman, R., J. Org. Chem., 53, 5547, (1988) which describes the diacylation of glycidyl tosylate in the presence of boron trifluoride etherate with a 76 percent yield. These Lewis acid catalyzed acylations give reasonable yields. However, in practice Lewis acid catalysts do not give good process economics on large scale due to catalyst costs and the need to use expensive, corrosion-resistant materials of construction.
U.S. Pat. No. 5,189,199 to Godleski discloses the addition of oxygen nucleophiles to 3,4-epoxy-1-butene catalyzed by ligated Pd(0) to prepare 1,4-disubstituted-2-butenes. However, applying this process for the production of 3,4-diacetoxy-1-butene using acetic anhydride requires additional separation because it predominately produces 1,4-diacetoxy-2-butene. Its application is limited in scope since the substrate must be an epoxide in direct conjugation with a carbon--carbon double bond.
Other processes are commonly used to prepare 1,2-diacetoxy esters such as the acetylation of 1,2-diols with acetic anhydride or acetyl chloride. However, when using acetic anhydride, acetic acid is co-produced and must be removed or recycled in the process. With acetyl chloride, an excess of an organic base is generally needed to remove the corrosive hydrochloric acid that is co-produced in the process. A large scale process utilizing acetyl chloride is inefficient since the hydrochloride salt needs to be treated to either regenerate the base or dispose of in a waste stream. In most cases, 1,2-diacetoxy esters are prepared from epoxides by acid catalyzed hydrolysis of epoxides to form the corresponding 1,2-dihydroxy derivative followed by acetylation of the hydroxyl groups with an equivalent amount of acetic anhydride and organic base, such as pyridine or 4-(N,N-dimethylamino)pyridine. This overall process requires two chemical steps and isolation of intermediates.
Thus, there exists a need whereby 1,2-diacetoxy esters may be produced from epoxides in a single step without the need to use corrosion resistant equipment, without the added costs of separating co-products and recovering by-products of the process and without the waste associated with the loss of catalysts due to thermal instability. Accordingly, it is to the provision of such improved processes for the preparation of 1,2-diacetoxy esters that the present invention is primarily directed.